In the field of implantable prosthetic devices, it is known that adhesion of a prosthesis to adjacent bone tissue can be promoted by providing a porous layer or area for ingrowth of bone. Bone tends to grow into interstices in the porous area, thereby securing the prosthesis. However, the metallic substrate of the prosthesis and the porous layer have different materials properties, particularly if different substances are used in the substrate and the porous layer. Adhesion between the substrate and the porous layer continues to be a matter of development in this field.
Several techniques have been proposed in the ongoing effort to adhere an appropriate porous layer to a substrate. For example, Hahn proposed in U.S. Pat. No. 3,605,123 that a dense base metal be coated with a porous film of the same material. The porous film was applied by a flame spray process, such as plasma flame processing. In U.S. Pat. No. 3,855,638, Pilliar described a prosthetic device having a solid metallic substrate with a porous coating adhered thereto. The porous coating was described as extending partially over the surface of the substrate. A slurry of metallic particles was applied to the substrate, dried and then sintered. Since both the substrate and the powder were involved in the sintering step, thermal stresses encountered by prior art methods were avoided. On different areas of the substrate, metallic particles of different sizes might be used, depending on whether a particular area of the prosthesis was expected to be adjacent hard or soft tissue.
Bokros, in U.S. Pat. No. 4,038,713, also employed sintering to affix coils in grooves of a prosthesis or to produce porous metallic tubes having some flexibility. Thereafter, in U.S. Pat. No. 4,206,516, Pilliar suggested the use of a thermally decomposable compound for the metallic particles. An example would have been titanium hydride particles. To affix the thermally decomposable compound to the substrate, two heating stages would be proposed, first for thermal decomposition and then for sintering.
In each of the patents mentioned above, the porous coat comprised metallic particles of a single size. In U.S. Pat. No. 4,524,539, Rowe, et al., described the preparation of a porous coating comprising multiple layers with progressively larger particles in subsequent layers. Hahn was a co-inventor of this patent. To deposit these layers on the substrate, a flame plasma process was described, similar to the process previously described by Hahn in his earlier Pat. No. 3,605,123 patent.